Though they are known for many years, plasma displays are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat colour panels of large size (out of the CRT limitations) and with very limited depth without any viewing angle constraints. Referring to the last generation of European CRT-TV, a lot of work has been done to improve its picture quality. Consequently, a new technology like Plasma has to provide a picture quality at least as good or even better than the old standard CRT-TV technology. On the one hand, the Plasma technology gives the possibility of “unlimited” screen size, of attractive thickness etc. but on the other hand it generates new kinds of artefacts that could degrade the picture quality. Most of these artefacts are different from those of CRT-TV pictures that make them more visible since people are used of seeing the old TV artefacts unconsciously.
The principle structure of a plasma cell in the so-called matrix plasma technology is shown in FIG. 1. Reference number 10 denotes a face plate made of glass, with reference number 11 a transparent line electrode is denoted. The back plate of the panel is referenced with reference number 12. There are 2 dielectric layers 13 for isolating face and back plate against each other. In the back plate column electrodes 14 are integrated being perpendicular to the line electrodes 11. The inner part of the cells consists of a luminance substance 15 (phosphorous) and separator 16 for separating the different coloured phosphorous substances (green 15a) (blue 15b) (red 15c). The UV radiation caused by the discharge is denoted with reference number 17. The light emitted from the green phosphorous 15a is indicated with an arrow having the reference number 18. From this structure of a PDP cell it is clear that there are three plasma cells necessary, corresponding to the three colour components RGB to produce the colour of a picture element (pixel) of the displayed picture.
The grey level of each R, G, B component of a pixel is controlled in a PDP by modulating the number of light pulses per frame period. The eye will integrate this time modulation over a period corresponding to the human eye response. The most efficient addressing scheme should be to address n times if the number of video levels to be created is equal to n. In case of the commonly used 8 bit representation of the video levels, a plasma cell should be addressed 256 times according to this. But this is not technically possible, since each addressing operation requires a lot of time (around 2 μs per line>960 μs for one addressing period>245 ms for all 256 addressing operations), which is more than the 20 ms available time period for 50 Hz video frames.
From the literature a different addressing scheme is known, which is more practical. According to this addressing scheme a minimum of 8 sub-fields (in case of an 8 bit video level data word) are used in a sub-field organization for a frame period. With a combination of these 8 sub-fields it is possible to generate the 256 different video levels. This addressing scheme is illustrated in FIG. 2. In this figure each video level for each colour component will be represented by a combination of 8 bits with the following weights:1/2/4/8/16/32/64/128
To realize such a coding with the PDP technology, the frame period will be divided in 8 lighting periods called sub-fields, each one corresponding to a bit in a corresponding sub-field code word. The number of light pulses for the bit “2” is double as for the bit “1” and so forth. With these 8 sub-periods it is possible, through sub-field combination, to build the 256 grey levels. The standard principle to generate this grey level rendition is based on the ADS (Address Display Separated) principle, where all operations are performed at different times on the whole display panel. At the bottom of FIG. 2 it is shown that in this addressing scheme each sub-field consists of three parts, namely an addressing period, a sustaining period and an erasing period.
In the ADS addressing scheme all the basic cycles follow one after the other. At first, all cells of the panel will be written (addressed) in one period, afterwards all cells will be lighted (sustained) and at the end all cells will be erased together.
The sub-field organization shown in FIG. 2 is only a simple example and there are very different sub-field organizations known from the literature with e.g. more sub-fields and different sub-field weights. Often more sub-fields are used to reduce moving artefacts and “priming” could be used on more sub-fields to increase the response fidelity. Priming is a separate optional period, where the cells are charged and erased. This charge can lead to a small discharge, i.e. can create background light, which is in principle unwanted. After the priming period an erase period follows for immediately quenching the charge. This is required for the following sub-field periods, where the cells need to be addressed again. So priming is a period, which facilitates the following addressing period, i.e. it improves efficiency of the writing stage by regularly exciting all cells simultaneously. The addressing period length can be equal for all sub-fields, also the erasing period length. However, it is also possible that the addressing period length is different for a first group of sub-fields and a second group of sub-fields in a sub-field organization. In the addressing period, the cells are addressed line-wise from line 1 to line n of the display. In the erasing period all the cells will be discharged in parallel in one shot, which does not take as much time as for addressing. The example in FIG. 3 shows the standard sub-field organisation with 8 sub-fields inclusive the priming operation. At one point in time there is one of these operations active for the whole panel.
This light emission pattern introduces new categories of image-quality degradation corresponding to disturbances of grey levels and colours. These will be defined as dynamic false contour since they correspond to the apparition of coloured edges in the picture when an observation point on the PDP screen moves. Such errors on a picture lead to the impression of strong contours appearing on homogeneous area like skin. The degradation is enhanced when the image has a smooth gradation and also when the light-emission period exceeds several milliseconds. In addition, the same problems occur on static images when observers are shaking their heads and that leads to the conclusion that such errors depend on the human visual perception. To understand a basic mechanism of visual perception of moving images, a simple case with a transition between the levels 128 and 127 moving at 5 pixel per frame, the eye following this movement, will be considered.
FIG. 4 represents in dark grey the lighting sub-fields corresponding to the level 128 and in grey, these corresponding to the level 127 with a standard 8 sub-field encoding.
On FIG. 4 one can follow the behaviour of the eye integration during a movement. The two extreme diagonal eye-integration-lines show the limits of the faulty perceived signal. Between them, the eye will perceive a lack of luminance that leads to the appearing of a dark edge shown in FIG. 5.
Instead of the standard 8 sub-field coding, we can choose a new coding scheme using more sub-fields as demonstrated in FIG. 6 showing a sub-field organisation with 12 sub-fields.
FIG. 7 shows the influence of the different sub-field organisation on the light generation in case of the 128/127 transition moving at 5 pixel per frame.
Furthermore, this figure shows the impact of the new coding on the false contour effect in the case of the 128/127 transition, in which the minimum video level perception on the retina is enhanced a lot from 0 to 123. Consequently, the number of sub-fields would have to be increased and then the picture quality in case of motion will be improved, too. Nevertheless an increasing of the sub-field number is limited according to the following relation:(1) nSF×NL×Tad+TLight≦TFramewhere nSF represents the number of sub-fields, NL the number of lines, Tad the duration to address one sub-field per line, TLight the lighting duration of the panel and TFrame the frame period. Obviously, an increasing of the sub-field number will reduce the time TLight to light the panel and consequently, will reduce the global brightness and contrast of the panel.
A first idea, called Bit-Line Repeat Principle (BLR), is to reduce, for some sub-fields named common sub-fields, the number of lines to be addressed by grouping k consecutive lines together. In that case the previous relation (1) is modified to the following one:                                                         n              CommonSF                        ×                          NL              k                        ×                          T              ad                                +                                    n              SpecificSF                        ×                          T              ad                                +                      T            Light                          ≤                  T          Frame                                    (        2        )            where ncommonSF represents the number of common sub-fields, nSpecificSF represents the number of specific sub-fields and k the number of consecutive lines having the same sub-fields in common.
The following example serves for demonstrating BLR-encoding in more detail with k=2. Assuming that only 9 sub-fields can be addressed with the current panel an acceptable contrast ratio will be achieved, but with 9 sub-fields, the false contour effect will stay very disturbing. Taking into account the previous sub-field coding of FIG. 6 and 7 that has a quite good behaviour concerning the false contour issue. In this coding scheme 6 independent sub-fields and 6 common sub-fields will be chosen, then the previous relation (2) becomes:                                           6            ×                          NL              2                        ×                          T              ad                                +                      6            ×            NL            ×                          T              ad                                +                      T            Light                          =                                            9              ×              NL              ×                              T                ad                                      +                          T              Light                                ≤                      T            Frame                                              (        3        )            which is equivalent to the relation in case of a 9 Sub-field coding. Consequently, with such a Bit-Line Repeat coding, we will artificially dispose of 12 sub-fields with the same amount of light pulses as with 9 sub-fields (same brightness and contrast). We will represent this example of Bit-Line Repeat coding as following:1-2-4-5-8-10-15-20-30-40-50-70 in which the underlined values represent the common sub-fields values. In that case, the values of these common sub-fields will be the same between each pixel of two consecutive lines since we have chosen k=2. Let us take an example of the values 36 and 51 located at the same horizontal position on two consecutive lines as shown in FIG. 8.
There are different possibilities to encode these values (the codes in brackets represent the corresponding codes for the 6 common sub-fields, with the LSB at the right side):                               36          =                    ⁢                                    30              _                        +                          4              _                        +                                          2                _                            ⁢                                                           ⁢                              (                100110                )                                                                                  =                    ⁢                                    30              _                        +            5            +                                          1                _                            ⁢                                                           ⁢                              (                100001                )                                                                                  =                    ⁢                      20            +                          15              _                        +                                          1                _                            ⁢                                                           ⁢                              (                010001                )                                                                                  =                    ⁢                      20            +            10            +            5            +                                          1                _                            ⁢                                                           ⁢                              (                000001                )                                                                                  =                    ⁢                      20            +            10            +                          4              _                        +                                          2                _                            ⁢                                                           ⁢                              (                000110                )                                                                                  =                    ⁢                      20            +                          8              _                        +            5            +                          2              _                        +                                          1                _                            ⁢                                                           ⁢                              (                001011                )                                                                                  =                    ⁢                                    15              _                        +            10            +                          8              _                        +                          2              _                        +                                          1                _                            ⁢                                                           ⁢                              (                011011                )                                                                                  =                    ⁢                                    15              _                        +            10            +            5            +                          4              _                        +                                          2                _                            ⁢                                                           ⁢                              (                010110                )                                                                                      51          =                    ⁢                      50            +                                          1                _                            ⁢                                                           ⁢                              (                000001                )                                                                                  =                    ⁢                      40            +            10            +                                          1                _                            ⁢                                                           ⁢                              (                000001                )                                                                                  =                    ⁢                      40            +                          8              _                        +                          2              _                        +                                          1                _                            ⁢                                                           ⁢                              (                001011                )                                                                                  =                    ⁢                      40            +            5            +                          4              _                        +                                          2                _                            ⁢                                                           ⁢                              (                000110                )                                                                                  =                    ⁢                                    30              _                        +            20            +                                          1                _                            ⁢                                                           ⁢                              (                100001                )                                                                                  =                    ⁢                                    30              _                        +            10            +                          8              _                        +                          2              _                        +                                          1                _                            ⁢                                                           ⁢                              (                101011                )                                                                                  =                    ⁢                                    30              _                        +            10            +            5            +                          4              _                        +                                          2                _                            ⁢                                                           ⁢                              (                100110                )                                                                                  =                    ⁢                      20            +                          15              _                        +            10            +            5            +                                          1                _                            ⁢                                                           ⁢                              (                010001                )                                                                                  =                    ⁢                      20            +                          15              _                        +            10            +                          4              _                        +                                          2                _                            ⁢                                                           ⁢                              (                010110                )                                                                                  =                    ⁢                      20            +                          15              _                        +                          8              _                        +            5            +                          2              _                        +                                          1                _                            ⁢                                                           ⁢                              (                011011                )                                                        
For this example one could find a way to encode these two values without any error (no loss of vertical resolution) in case of Bit-Line Repeat (same coding on common sub-fields=same values in brackets):
36=30 + 4 + 2and51=30 + 10 + 5 + 4 + 236=30 + 5 + 1and51=30 + 20 + 136=20 + 15 + 1and51=20 + 15 + 10 + 5 + 136=20 + 10 + 5 + 1and51=50 + 136=20 + 10 + 5 + 1and51=40 + 10 + 136=20 + 10 + 4 + 2and51=40 + 5 + 4 + 236=20 + 8 + 5 + 2 + 1and51=40 + 8 + 2 + 136=15 + 10 + 8 + 2 + 1and51=20 + 15 + 8 + 5 + 2 + 136=15 + 10 + 5 + 4 + 2and51=20 + 15 + 10 + 4 + 2
Nevertheless, there are some cases in which an error has to be made due to the reduced flexibility in encoding produced by the need to have the same coding for each common sub-field. For instance, the values 36 and 52 have to be replaced by 36 and 51 or 37 and 52 to have the same code on common sub-fields. In addition, since there are common values between two consecutive lines, the biggest difference between these two lines can only be achieved through the non-common sub-field. That means, for our example, that the maximum vertical transition in the picture is limited to 195. This limitation introduces a reduction of the vertical resolution combined with new artefacts studied below.
The relation (2) presents a main condition of the global BLR concept based on k (k≧2) common lines. For the following explanations, it is assumed that we dispose of 7 standard sub-fields and k=6 is chosen. FIG. 9 illustrates this concept. The six pixels located at the same horizontal position but on six consecutive lines will be encoded with the same common sub-fields but their specificity will be encoded with the specific sub-fields.
The following BLR code with 256 levels will be used as example:1-2-4-5-8-10-15-20-30-40-50-70 
The underlined values represent the common values. This code has the time cost of 7 standard sub-fields (6 specific with normal addressing time +6 common with a sixth of the addressing time) but improves the grey-scale rendition as the false contour behaviour of the panel. The maximal transition possible in these 6 common lines is limited by the sum of the specific values (Σ=195). Consequently, there is still a loss of resolution in the picture but this can be optimised with a dedicated encoding algorithm. The precise specification of the BLR encoding principle has been presented in previous European Patent Applications (EP-A-0874349, EP-A-0874348, EP-A-0945846, WO-A-00/25291, EP-A-1058229 and PCT/FR00/02498). Nevertheless, the following gives an overall presentation of the encoding algorithm:    {circle around (1)} In the amount of k values, select the smallest and biggest values Vmax and Vmin.    {circle around (2)} Modify these two values to have a difference D=(Vmax′-Vmin′) as multiple of five.    {circle around (3)} Modify all values which have a difference with Vmin′ which is higher than the maximal available transition (Σ of specific values=SPEmax) to Vmin′+SPEmax. These new values will be the new highest video value Vmax″.    {circle around (4)} Encode the new maximal value as a standard video value without taking into account the BLR concept.    {circle around (5)} Check that the sum of all common values from Vmax″ is smaller than Vmin′. If it is not the case, replace the common value from Vmax″ by the common values needed to encode Vmin′. These common values will be used for the encoding of all values. The code will be called COM_PART since it corresponds to the code based on common sub-fields (i.e. common part) only.    {circle around (6)} Encode all the values taking into account this common part COM_PART.
An example shown in FIG. 10 will help to illustrate this algorithm.
The following encoding steps are performed:    {circle around (1)} Vmax=128 and Vmin=52.    {circle around (2)} Vmax′=127 and Vmin′=52 with a difference D=(Vmax′−Vmin′)=75=5×15.    {circle around (3)} Nothing to do.    {circle around (4)} 127=1+2+4+5+10+15+20+30+40    {circle around (5)} COM_PART=1+2+4+15+30=52. In this example, COMP_PART (52)≦Vmin′(52)    {circle around (6)} Encoding of all values:            521+2+4+15+30=52 [no error]        601+2+4+10+15+30=62 [error=2]        861+2+4+5+10+15+20+30=87 [error=1]        1151+2+4+5 +15+20+3040=117 [error=2]        1281+2+4+5 +10+15+20+30+127 [error=1]        821+2+4+10+15+20+30=82 [no error]        
In the previous example, one can see that the lack of freedom coming from the BLR algorithm will introduce some errors in the encoding of the original values. This can lead to the introduction of a new noise in the picture that is one of the compromises needed to improve the grey-scale rendition as well as the false contour behaviour. Nevertheless, the most artefacts are introduced by the limitation in the vertical resolution.